Telephone handsets, especially mobile phones, are often operated in noise polluted environments. Microphone(s) of the handset being designed to pick up the user's voice signal unavoidably pick up environmental noise, which leads to a degradation of communication comfort. Several methods are known to improve communication quality in such use cases. Normally, communication quality is improved by attempting to reduce the noise level without distorting the voice signal. There are methods that reduce the noise level of the microphone signal by means of assumptions about the nature of the noise, e.g. continuity in time. Such single-microphone methods as disclosed, e.g., in German patent DE 199 48 308 C2 achieve a considerable level of noise reduction. However, the voice quality degrades if there is a high noise level, and a high noise suppression level is applied.
Other methods use an additional microphone for further improvement of the communication quality. Different geometries can be distinguished, which are addressed as methods with “symmetric microphones” or “asymmetric microphones”. Symmetric microphones usually have a spacing as small as 1-2 cm between the microphones, where both microphones pick up the voice signal in a rather similar manner and there is no principle distinction between the microphones. Such methods as disclosed, e.g., in German patent DE 10 2004 005 998 B3 require information about the expected sound source location, i.e. the position of the user's mouth relative to the microphones, since geometric assumptions are the basis of such methods.
Further developments are capable of in-system adaptation, wherein the algorithm applied is able to cope with different and a-priori unknown positions of the sound source. However, such adaption requires noise-free situations to “calibrate” the system as disclosed, e.g. in German patent application DE 10 2010 001 935 A1.
“Asymmetric microphones” typically have greater distances of around 10 cm, and they are positioned in a way that the level of voice pick-up is as distinct as possible, i.e. one microphone faces the user's mouth, the other one is placed as far away as possible from the user's mouth, e.g. at the top edge or back side of a telephone handset. The goal of the asymmetric geometry is a difference of preferably approximately 10 dB in the voice signal level between the microphones. The simplest method of this kind just subtracts the signal of the “noise microphone” (away from user's mouth) from the “voice microphone” (near user's mouth), taking into account the distance if the microphones. However since the noise is not exactly the same in both microphones and its impact direction is usually unknown, the effect of such a simple approach is poor.
More advanced methods try to estimate the time difference between signal components in both microphone signals by detecting certain features in the microphone signals in order to achieve a better noise reduction results, cf. e.g., WO 2003/043374 A1. However, feature detection can get very difficult under certain conditions, e.g. if there is a high reverberation level. Removing such reverberation is another aspect of 2-microphone methods as disclosed, e.g., in WO2006/041735 A2, in which spectra-temporal signal processing is applied.
Therefore, none of the methods or systems known in the art allow robust improvement of the communication quality for a wide range of ambient noise conditions.